Apparatus for sharpening rotary cutters for tobacco or the like

ABSTRACT

The cutting edges of knives which are mounted on a rotary holder of a tobacco shredding machine are sharpened by the front surface or surfaces of one or more crown grinding wheels mounted on one or more carriages which are reciprocable in parallelism with the axis of the holder and dwell for selected intervals of time at the respective axial ends of the holder. The front surface of each grinding wheel is reconditioned by a discrete diamond which is pivotable about a fixed axis at the respective axial end of the holder. Each grinding wheel is fed axially toward the holder to compensate for wear and to maintain the respective front surface at a fixed distance from the axis of the holder. Each diamond can be set in motion in automatic response to completion of movement of the respective carriage toward the respective axial end of the holder.

[ July 31, 1973 APPARATUS FOR SHARPENING ROTARY CUTTERS FOR TOBACCO OR THE LIKE [75] Inventors: Uwe Elsner; Werner Komossa, both of Hamburg, Germany [73] Assignee: Hauni-Werke Korber & Co. KG,

Hamburg, Germany [22] Filed: Mar. 26, 1971 [21] Appl. No.: 128,475

[ 30] Foreign Application Priority Data 7/1949 Bloom 51/249 X 11/1967 Postula 51/249 Primary Examiner-Donald G. Kelly Att0rneyMichael S. Striker [57] ABSTRACT The cutting edges of knives which are mounted on a rotary holder of a tobacco shredding machine are sharpened by the front surface or surfaces of one or more crown grinding wheels mounted on one or more carriages which are reciprocable in parallelism with the axis of the holder and dwell for selected intervals of time at the respective axial ends of the holder. The front surface of each grinding wheel is reconditioned by a discrete diamond which is pivotable about a fixed axis at the respective axial end of the holder. Each grinding wheel is fed axially toward the holder to compensate for wear and to maintain the respective front surface at a fixed distance from the axis of the holder. Each diamond can be set in motion in automatic response to completion of movement of the respective carriage toward the respective axial end of the holder.

11 Claims, 3 Drawing Figures PATENTEDJUUHSB 3'748786 sum 1 or 2 FTTORNEV APPARATUS FOR SIIARPENING ROTARY CUTTERS FOR TOBACCO OR THE LIKE BACKGROUND OF THE INVENTION The present invention relates to improvements in apparatus for sharpening the cutting edge or edges of one or more rotary cutters, especially for sharpening the cutting edges of knives on the rotary holders of tobacco shredding or like machines. More particularly, the invention relates to improvements in sharpening apparatus employing rotary grinding wheels which are caused to move along the circular path of one or or more cutting edges.

US. Pat. No. 3,328,924 to Ward discloses a sharpening apparatus wherein the rim of the grinding wheel is hollowed to match the curvature of the path for the orbiting cutting edges of knives which are mounted in a rapidly rotating holder or head of a tobacco shredding machine. The rim is reconditioned by a dresser secured to an arm which is mounted at one axial end of the holder to pivot about an axis which is close to the holder axis. The radius of the path for the diamond is adjustable. A drawback of such sharpening apparatus is that the area of the working surface (rim) of the grinding wheel decreases with progressing wear on the grinding wheel and that the peripheral speed of the working surface also decreases in response to such wear. Therefore, the apparatus must be provided with a complicated system for increasing the rotational speed of the grinding wheel in response to wear. Futhermore, the grinding wheel must be replaced at frequent intervals because its sharpening action is unsatisfactory when the diameter of the rim decreases below a relatively large value. Frequent replacement of the grinding wheel is particularly undesirable in modern high-speed cutting or shredding machines wherein a short-lasting interruption of operation can cause substantial losses in output, as well as in cutting or shredding machines in production lines which are composed of a large number of machines because an interruption of operation of a single machine often necessitates stoppage of all machines in the production line. 4

A futher drawback of the just described sharpening apparatus is the complicatedness of the adjusting mechanism for the diamond. Also, the diamond is not readily accessible when the shredding machine is in use.

SUMMARY OF THE INVENTION An object of the invention is to provide a novel and improved sharpening apparatus which can be used in machines for shredding of tobacco or the like and wherein the sharpening tool or tools must be replaced at less frequent intervals than in presently known apparatus.

Another object of the invention is to provide a sharpening apparatus wherein the grinding or sharpening tool or tools must be replaced only when they are reduced to a small fraction of their original size.

A further object of the invention is to provide a sharpening apparatus wherein the grinding tool or tools can be driven at a constant speed and wherein the area of working surfaces on the grinding tools does not change with progressing wear on the tools.

An additional object of the invention is to provide novel reconditioning means for the working surfaces of tools in apparatus for sharpening the cutting edges of LII orbiting knives for the severing of tobacco, filter material or the like.

Still another object of the invention is to provide a sharpening apparatus whose components are more readily accessible than in presently known apparatus and which can be installed in existing tobacco shredding or like machines.

A further object of the invention is to provide the sharpening apparatus with simple, reliable and readily accessible reconditioning means for the working surface or surfaces of one or more sharpening tools.

Another object of the invention is to provide the apparatus with means for facilitating the selection of intervals at which the working surface or surfaces of one or more sharpening tools are to be subjected to a reconstituting or dressing treatment.

Th improved apparatus is used in a machine wherein a rotary holder supports at least one knife or cutter having an exposed cutting edge which travels along a circular path in response to rotation of the holder. The apparatus comprises a carriage which is movable in parallelism with the axis of the holder and a grinding wheel rotatably mounted in the carriage and having a front surface adjacent to the circular path so as to periodically sharpen the cutting edge while the carriage is in motion.

In accordance with another feature of the invention, the apparatus further comprises reconditioning means including a dresser (preferably a diamond) which is pivotable about a predetermined axis to thereby recondition the front surface of the grinding wheel. The dresser is adjacent to one axial end of the holder and the carriage is caused to dwell in a predetermined position at the one end of the holder between successive movements of the carriage while the dresser pivots about the respective axis to thereby treat the front surface of the grinding wheel.

The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The improved sharpening apparatus itself, however, both as to its construction and its mode of operation, together with additional features and advantages thereof, will be best understood upon perusal of the following detailed description of certain specific embodiments with reference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic end elevational view of a sharpening apparatus which embodies one form of the invention and is used in a tobacco shredding machine;

FIG. 2 is a fragmentary plan view of the structure shown in FIG. 1; and

FIG. 3 is a schematic rear elevational view of a second sharpening apparatus with two grinding wheels and with discrete reconditioning means for the working surfaces of the grinding wheels.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIG. 1, there is shown a portion of a tobacco shredding machine having a feeding unit 1 which serves to advance a cake of compacted tobacco leaves into the range of cutting edges 8 on elongated blade-like cutters or knives 6 mounted in a rotary holder or head 2. The character 3 denotes a sharpening apparatus which serves to sharpen the cutting edges 8 while the holder 2 rotates to move successive knives 6 past the outlet of a mouthpiece or spout 17 which feeds the leading end of the tobacco cake into the range of the knives. The sharpening apparatus 3 comprises a crown grinding wheel 7 having a front surface or active surface 11 which can be treated at regular intervals by a reconditioning unit 4 having a dresser in the form of a diamond 9.

In addition to the spout 17, the feeding unit 1 comprises two endless belts or chains 12, 13 which are respectively trained over pulleys or sprocket wheels 14, 16 and are driven to respectively advance their lower and upper stretches in the directions indicated by arrows 21 and 22. Such stretches define a channel whose height decreases in a direction toward the spout 17. The latter comprises an upper portion 18 which is adjacent to the front pulley or sprocket wheel 14 and a lower portion 19 which is adjacent to the front pulley or sprocket wheel 16. The exact mode of operation of the feeding unit 1 forms no part of the present invention.

The holder 2 is rotatable about the axis of a horizontal shaft 23 and is driven by a motor (not shown) to rotate in a counterclockwise direction as indicated by the arrow 24. The shaft 23 is mounted in fixed bearings. The holder 2 is hollow and accommodates a customary advancing mechanism which moves the knives 6 outwardly so as to compensate for the wear on the cutting edges 8. Reference may be had to U.S. Pat. No.

3,117,602 which discloses one of several suitable advancing mechanisms capable of being incorporated in the holder 2. The advancing mechanism insures that the cutting edges 8 travel along a circular path having a radius A slightly exceeding the radius of the holder 2. The wear on the cutting edges 8 is due to their tobaccoshredding action and particularly to periodic sharpening by the front surface of active surface 11 of the grinding wheel 7.

The grinding wheel 7 is mounted on a shaft 26 which is rotatable by a motor (not shown) mounted in or on a carriage or housing 27 of the sharpening apparatus 3. The shredding machine comprises guide means or ways 28 extending in parallelism with the axis of the holder 2 and serving to guide the carriage 27 for movement to and from a predetermined position of dwell (shown in FIG. 2) between two successive movements. In such position of the carriage 27, the grinding wheel 7 is adjacent to one axial end of th holder 2 and the front surface 11 of the grinding wheel is in the range of the diamond 9. The means for moving the carriage 27 along the ways 28 to and from the position of FIG. 2 (see the double-headed arrow 29) may comprise a hydraulic or pneumatic cylinder and piston unit having a piston rod 128 a portion of which is shown in FIG. 2. The carriage 27 is preferably caused to dwell in the position of FIG. 2 for an interval of preselected length upon completion of a full forward stroke (downwardly, as viewed in FIG. 2) and an immediately following return stroke (upwardly, as viewed in FIG. 2). The means for rotating the shaft 26 for the grinding wheel 7 at a high speed preferably includes an electric motor which is mounted in or on the carriage 27. The annular front surface 11 of the grinding wheel 7 is of concave outline, as considered in the radial direction of the grinding wheel, and its center of curvature is preferably located exactly on or very close to the axis of the shaft 23. If one disregards the concavity of the front surface 11, each radially extending strip or line of this surface makes with the general plane of the grinding wheel 7 (i.e., with a plane 31 which is normal to the axis of the shaft 26) an acute angle alpha. The just described configuration of the front surface 11 insures that a relatively small and lightweight grinding wheel 7 can be used for an extended period of time and that its rotational speed need not be changed because the overall area of the front surface 11 remains at least substantially constant even though the wear on the grinding wheel 7 progresses in the axial direction of the shaft 26. This shaft is movable axially at certain intervals or continuously so as to compensate for wear on the grinding wheel 7, i.e., to maintain the front surface 11 at a constant distance from the periphery of the holder 2. Since the center of curvature of the front surface 11 is located on or very close to the axis of the shaft 23, successiive cutting edges 8 are sharpened in a fully automatic way when the holder 2 rotates and the carriage 27 moves toward or from the position shown in FIG. 2. Furthermore, the front surface 11 treats each part of each cutting edge 8 during each cycle of operation of the sharpening apparatus 3, Le, during movement of the carriage 27 away from and back to the position of FIG. 2.

Th aforementioned reconditioning unit 4 further comprises a pivot 32 which is fixedly mounted at a predetermined distance C from the axis of the shaft 23 and carries an elongated arm 33 for the diamond 9. The arm 33 is caused to pivot back and forth (as indicated by the double-headed arrow 34) when the carriage 27 dwells in the position shown in FIG. 2. The diamond 9 then treats all parts of the rotating front surface 11 on the grinding wheel 7; it performs a corrective and a smoothing action. The axis of the pivot 32 is parallel to the axis of the shaft 23 and is normal to the axis of the shaft 26. The transmission which causes the arm 33 to pivot back and forth (arrow 34) when the carriage 27 assumes the position of FIG. 2 is of known design and is not shown in the drawing. Such transmission can be started in automatic response to movement of the car riage 27 to the postion of FIG. 2 and can be arrested with a predetermined delay which is necessary to insure satisfactory treatment of the entire front surface 11. The distance B between the axis of the pivot 32 and the front surface 11 of the grinding wheel 7 when the carriage 27 assumes the position of FIG. 2 slightly exceeds the radius A of the circular path for the cutting edges 8.

It will be noted that the pivot 32 for the arm 33 of the diamond 9 is outwardly adjacent to the circular path of the cutting edges 8. This is desirable in order to insure ready accessiblity of the entire reconditioning unit 4 regardless of whether the shredding machine is in operation or at a standstill. The unit 4 is very simple and compact, mainly because the axis of the pivot 32 is preferably fixed If desired, the structure of FIGS. 1 and 2 may include two reconditioning units 4, one at each axial end of the holder 2. The means for moving the carriage 27 along the ways 28 is then designed in such a way that the carriage dwells for a certain interval of time in each of its two end positions whereby the front surface 11 is treated by one of the diamonds in one end position and by the other diamond in the other end position of the carriage 27. As will be explained in connection with FIG. 3, the arm or arms 33 can be pivoted in automatic response to each n-th movement of the carriage 27 to the respective end position, n being a whole number including one.

Though the drawing shows a piston rod 128 which forms part of a hydraulic or pneumatic cylinder and piston unit, it is clear that the carriage 27 can be reciprocated by other suitable means, such as an electric motor. A hydraulic cylinder and piston unit has been found to be especially suited because it renders it possible to select the strokes of the carriage 27 with a high degree of accuracy.

Since the distance B slightly exceeds the radius A of the path for the cutting edges 8, the front surface 11 of the grinding wheel 7 is shaped by the diamond 9 in such a way that it is always in a mere point contact with a cutting edge 8. This contributes to accuracy of the sharpening action.

The operation:

The feeding unit 1 comprises a chute (not shown) which supplies tobacco leaves into the channel between the lower stretch of the upper belt or chain 12 and the upper stretch of the lower belt of chain 13. The leaves are condensed to form a cake which passes through the spout l7 and into the range of the cutting edges 8 of knives 6. The holder 2 is rotated in the direction indicated by the arrow 24. The tobacco shreds descend into a collecting receptacle or onto a conveyor, not shown, for delivery into a cigarette rod making machine. The holder 2 is driven at a very high speed so that the cutting edges 8 require frequent sharpening which is carried out by the front surface 11 of the grinding wheel 7. The sharpening action takes place while the carriage 27 moves along the ways 28, i.e., in parallelism with the axis of the shaft 23. The grinding wheel 7 is rotated at a high speed and, as the carriage 27 moves toward or away from the position of FIG. 2, the front surface 11 treats each of the cutting edges 8 at regularly recurring intervals in such a way that each of the cutting edges 8 is sharpened in its entirety, namely, all the way from the one to the other axial end of the holder 2. 1

When the carriage 27 completes a return stroke (upwardly, as viewed in FIG. 2), it comes to rest for a period of time which is sufficient to allow for reconditioning of the front surface 11 by the diamond 9. As mentioned before, the area of the surface 11 reamins unchanged irrespective of the wear on the grinding wheel 7 because the surface 11 is a front surface of the grinding wheel. Therefore, the sharpening action of the surface 11 upon the cutting edges 8 is not dependent on the extent of wear upon the grinding wheel 7, and the latter can be driven at a constant speed. The area of the surface 11 would begin to decrease only if the grinding wheel 7 were pennitted to wear away to such an extent that the outer edge of the surface 11 would reach the rear end face of the grinding wheel. As mentioned above, the grinding wheel 7 is fed axially (away from the carriage 27) at regular or irregular intervals or continuously so as to insure that the distance B between the front surface 11 and the axis of the pivot 32 remains unchanged.

The treatment of the surface 11 by the diamond 9 is needed to remove the metallic fragments which accumulate on the surface 11 during treatment of the cutting edges 8 as well as to insure that the configuration of the surface 11 remains unchanged. Thus, the center of curvature of the surface 1 1 should remain on or very close to the axis of the shaft 23. The diamond 9 can treat the front surface 11 while the carriage 27 dwells in the position of FIG. 2 and while the arm 33 pivots back and forth as indicated by the arrow 34. The grinding wheel 7 continues to rotate at a high speed while the carriage 27 dwells in the position of FIG. 2.

The distance B slightly exceeds the radius A of the circular path of the cutting edges 8 and the distance C is selected in such a way that the cutting edges 8 cannot strike against the front surface 11 when the grinding wheel 7 moves with the carriage 27. The just described relationship of the distance B to the radius A and the fact that the distance C exceeds the distance B insures smooth engagement between successive cutting edges 8 and the front surface 11. Since the holder 2 rotates at a high speed, and since the grinding wheel 7 is also rotated at a high speed, each increment of each cutting edge 8 comes into contact with a different portion of the front surface 11 to thus insure uniform wear on the grinding wheel.

FIG. 3 illustrates the sharpening apparatus and the reconditioning means in a second tobacco shredding machine having a rotary holder or head 51 with equidistant longitudinally extending knives having cutting edges 52. The sharpening apparatus of FIG. 3 comprises two discrete sharpening units 53, 54 respectively having grinding wheels 56, 59 which are mounted on carriages or housings 57, 61 movable in parallelism with the axis of the holder 51 as indicated by the double-headed arrows 63 and 64. The configuration and the manner of mounting of each of the grinding wheels 56, 59 are the same as described in connection with the grinding wheel 7, i.e., each of these grinding wheels is fed axially and has an active surface which consitutes a front surface and is of slightly concave outline so as to have its center of curvature on or close to the axis of the holder 51. The shaft (not shown) for the grinding wheel 56in the sharpening unit 53 is driven by an electric motor 58 which is mounted on the carriage 57 and is connected with an energy source by means of a flexible cable 66.

The sharpening unit 54 is substantially identical with the sharpening unit 53. It comprises a first motor 62 which consitutes the prime mover of means for causing the carriages 57, 61 to move back and forth in parallelism with the axis of the holder 51, and a second electric motor 158 which serves to rotate the grinding wheel 59 and is located behind the motor 62, as viewed in FIG. 3. The cable 67 serves to connect the motors 158, 62 with an energy source.

The carriages 57, 61 are rigidly but preferably separably connected to each other by a coupling bar or rod 68 so that the carriage 57 shares all movements of the carriage 61. The guide means for the carriages 57, 61 includes tie rods 69, 71 which are secured to upright frame members 73, 74. The means for moving the carriages 57, 61 along the tie rods 69, 71 includes the aforementioned motor 62 and a toothed rack 72 which is parallel to the tie rods and meshes with a pinion (not shown) on the output shaft of the motor 62. The ends of the rack 72 are secured to the frame members 73, 74. These frame members respectively support limit switches 76, 77 which respectively serve to start the motors 88, 96 of two reconditioning units 78, 79. Th units 78, 79 are adjacent to the opposite axial ends of the holder 51 and respectively comprise horizontal pivots 86, 93 for arms 83, 91 which carry dressers in the form of diamonds 84, 92. The motors 88,96 respectively serve to pivot the arms 83, 91 back and forth (preferably by way of the respective pivots 86, 93) while the respective grinding wheels 56, 59 are in the range of the corresponding diamonds 84, 92. The operative connections between the limit switches 76, 77 and the respective motors 88, 96 are indicated by phantom lines, as at 89 and 97. These connections contain adjustable counters 81, 82 which can start the motors 88, 96 in response to signals from the respective limit switches 76, 77. The pivots 86, 93 are respectively mounted in stationary bearing members 87, 94.

The main difference between the structures of FIGS. 1-2 and FIG. 3 is that each of the grinding wheels 56, 59 serves to sharpen only one-half of each of the cutting edges 52. Thus, the strokes of the carriages 57, 61 are shorter than the strokes of the carriage 27. When the carriage 61 reaches the right-hand end of its stroke and actuates the limit switch 77, the carriage 57 occupics the space shown in FIG. 3 as being occupied by the carriage 61. The direction of operation of the motor 62 is automatically reversed when the carriage 57 actuates the limit switch 76 or when the carriage 61 actuates the limit switch 77. The reversals in the direction of operation of the motor 62 takes place with a preselected delay in order to enable the diamond 84 and 92 to treat the front surface of the grinding wheel 56 or 59 before the respective carriage 57 or 61 resumes its movement to leave the respective position of dwell at the leftor right-hand axial end of the holder 51.

When the motor 62 is on and the carriages 57, 61 move along the tie rods 69, 71, the front surface of the grinding wheel 56 sharpens the left-hand portions of the cutting edges 52 while the front surface of the grinding wheel 59 sharpens the right-hand portions of such cutting edges. The speed of movement of the carriages 57, 61 is preferably synchronized with the rotational speed of the holder 51. FIG. 3 shows the carriages 57, 61 in their left-hand end positions. Thus, the front surface of the grinding wheel 56 is in the range of the diamond 84 which is assumed to move back and forth in response to movement of the arm 83 about the axis of the pivot 86. The motor 88 is assumed to be on because the carriage 57 engages the limit switch 76. The counter 81 in the operative connection 89 can serve as a time delay means to stop the motor 88 with a predetermined delay which suffices to insure satisfactory reconditioning of the front surface on the grinding wheel 56 as well as a means for selecting the frequency at which the motor 88 is started. For example, the counter 81 can include a movable member (such as the slider of a potentiometer) which is advanced by a step in response to each acutation of the limit switch 76 and causes the motor 88 to pivot the arm 83 for the dia mond 84 in response to each n-th actuation of the limit switch 76 (n being a whole number including one). The operation of the counter 82 in the operative connection 97 between the limit switch 77 and motor 96 is preferably identical with that of the counter 81. By way of example, the counters 81, 82 can be adjusted in such a way that the motors 88, 96 are respectively started (and continue to operate for predetermined intervals of time) in response to each fifth actuation of the limit switches 76, 77.

A very important advantage of the improved sharpening apparatus is that the grinding wheels 7, 56, 59 can stand much longer wear than presently employed grinding wheels wherein the peripheral surface or rim serves to sharpen the orbiting cutting edges on a rotary holder. As explained hereinbefore in connection with the grinding wheel 7, the area of the front surface 11 remains unchanged even if the major part of the grinding wheel is used up, as long as the diameter of the outer edge of the front surface does not decrease below the diameter of a fresh grinding wheel. The same holds true for the grinding wheels 56, 59 of the sharpening apparatus shown in FIG. 3.

' Another important advantage of the sharpening apparatus is that the rotational speed of the grinding wheels need not be changed in response to progressing wear. This is also due to the fact that the cutting edges are treated by the front surfaces of the respective grinding wheels so that the area of the front surfaces is practically unchanged while the wear on the respective grinding wheels progresses.

A further important advantage of the immproved apparatus is that the diamonds 9, 84 or 92 can be mounted for pivotal movement about fixed axes and that all parts of the reconditioning units 3, 78, 79 are readily accessible at all times because they are mounted at the axial ends of the rotary knife holders. A specific advantage of the structure shown in FIG. 3 is that the grinding wheels 56, 59 must be replaced at less frequent intervals than the grinding wheel 7 because the wear on such grinding wheels is half the wear on a single grinding wheel.

The counters 81, 82 of FIG 3 render it possible to reduce the wear on the grinding wheels and to prolong the useful life of the diamonds 84, 92 because they enable the person in charge to reduce the frequency of the reconditioning treatment if the cutting edges 52 can be maintained in satisfactory condition with less frequent reconditioning of front surfaces on the grinding wheels 56 and 59. Of course, the structure of FIGS. 1 and 2 can also include a counter or other adjustable means which can be regulated to enable the person in charge to start the pivotal movement of the arm 33 for the diamond 9 whenever the carriage 27 completes each second, third, etc., movement to the position of FIG. 2.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features which fairly constitute essential characteristics of the generic aspects of our contribution to the art and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the claims.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims:

1. In an apparatus of the character indicated, particularly in a tobacco shredding machine, a combination comprising a rotary holder; at least one knife mounted in said holder and having an exposed cutting edge arranged to travel along a circular path in response to rotation of said holder; a carriage movable in parallelism with the axis of said holder; and a grinding wheel rotatably mounted in said carriage and having a conical active surface provided at one axial end of said grinding wheel and being adjacent to said path so as to periodically sharpen said cutting edge during movement of said carriage, said active surface having a concave outline and subtending an acute angle with a plane which is normal to the axis of said grinding wheel, the diameter of said one axial end of said grinding wheel decreasing in a direction toward said path.

2. A combination as defined in claim 1, further comprising reconditioning means including a dresser pivotable about a predetermined axis to thereby recondition said active surface, said dresser being adjacent to one axial end of said holder and said carriage being arranged to dwell in a predetermined position at said one end of said holder between successive movements thereof while said dresser pivots about the respective axis to thereby treat said active surface.

3. A combination as defined in claim 2, wherein said predetermined axis is at least substantially parallel with the axis of said holder.

4. In an apparatus of the character indicated, particularly in a tobacco shredding machine, a combination comprising a rotary holder; at least one knife mounted in said holder and having an exposed cutting edge arranged to travel along a circular path in response to rotation of said holder; a carriage movable in parallelism with the axis of said holder; a grinding wheel rotatably mounted in said carriage and having a front surface adjacent to said path so as to periodically sharpen said cutting edge during movement of said carriage; and reconditioning means including a dresser pivotable about a predetermined axis to thereby recondition said front surface, said dresser being adjacent to one axial end of said holder and said carriage being arranged to dwell in a predetermined position at said one end of said holder between successive movements thereof while said dresser pivots about the respective axis to thereby treat said front surface, said predetermined axis being at least substantially parallel with the axis of said holder and the shortest distance between said predetermined axis and the axis of said holder exceeding the radius of said path.

5. A combination as defined in claim 2, wherein said predetermined axis is fixed.

6. A combination as defined in claim 2, wherein said predetermined axis is and crosses in space with normal to the axis of said grinding wheel.

7. In an apparatus of the character indicated, particularly in a tobacco shredding machine, a combination comprising a rotary holder; at least one knife mounted in said holder and having an exposed cutting edge arranged to travel along a circular path in response to rotation of said holder; a carriage movable in parallelism with the axis of said holder; a grinding wheel rotatably mounted in said carriage and having a front surface adjacent to said path so as to periodically sharpen said cutting edge during movement of said carriage; reconditioning means including a dresser pivotable about a predetermined axis to thereby recondition said front surface, said dresser being adjacent to one axial end of said holder and said carriage being arranged to dwell in a predetermined position at said one end of said holder between successive movements thereof while said dresser pivots about the respective axis to thereby treat said front surface; and synchronizing means for effecting pivotal movements of said dresser in response to movement of said carriage to said predetermined position,said front surface of said grinding wheel being disengaged from said cutting edge in said predetermined position of said carriage.

8..A combination as defined in claim 1, further comprising a second carriage movable in parallelism with the axis of said holder and a second grinding wheel rotatably mounted on said second carriage and having a conical active surface adjacent to said path so as to pcriodically sharpen said cutting edge during movement of said second carriage.

9. ln an apparatus of the character indicated, particularly; in a tobacco shredding machine, a combination comprising a rotary holder; at least one knife mounted in said holder and having an exposed cutting edge arranged to travel along a circular path in response to retation of said holder; a first carriage movable in para]- lelisrn with the axis of said holder; a first grinding wheel rotatably mounted in said carriage and having a front surface adjacent to said path so as to periodically sharpen said cutting edge during movement of said carriage;"a second carriage movable in parallelism with the axis of said holder; a second grinding wheel rotatably mounted in said second carriage and having a front surface adjacent to said path so as to periodically sharpen said cutting edge during movement of said second carriage;,and discrete reconditioning means for each of said grinding wheels, each of said reconditioning means including a dresser pivotable about a predetermined axis to thereby recondition the front surface of the respective grinding wheel, said dressers being adjacent to the opposite axial ends of said holder and said carriages being arranged to dwell in predetermined positions at the respective axial ends of said holder between successive movements thereof while said dressers pivot about the respective axes to thereby treat the corresponding front surfaces.

10. A combination as defined in claim 9, further comprising coupling means connecting said carriages for simultaneous movement with reference to said holder.

11. A combination as defined in claim 1, wherein the radius of curvature of said active surface at least equals the radius of said circular path. 

1. In an apparatus of the character indicated, particularly in a tobacco shredding machine, a combination comprising a rotary holder; at least one knife mounted in said holder and having an exposed cutting edge arranged to travel along a circular path in response to rotation of said holder; a carriage movable in parallelism with the axis of said holder; and a grinding wheel rotatably mounted in said carriage and having a conical active surface provided at one axial end of said grinding wheel and being adjacent to said path so as to periodically sharpen said cutting edge during movement of said carriage, said active surface having a concave outline and subtending an acute angle with a plane which is normal to the axis of said grinding wheel, the diameter of said one axial end of said grinding wheel decreasing in a direction toward said path.
 2. A combination as defined in claim 1, further comprising reconditioning means including a dresser pivotable about a predetermined axis to thereby recondition said active surface, said dresser being adjacent to one axial end of said holder and said carriage being arranged to dwell in a predetermined position at said one end of said holder between successive movements thereof while said dresser pivots about the respective axis to thereby treat said active surface.
 3. A combination as defined in claim 2, wherein said predetermined axis is at least substantially parallel with the axis of said holder.
 4. In an apparatus of the character indicated, particularly in a tobacco shredding machine, a combination comprising a rotary holder; at least one knife mounted in said holder and having an exposed cutting edge arranged to travel along a circular path in response to rotation of said holder; a carriage movable in parallelism with the axis of said holder; a grinding wheel rotatably mounted in said carriage and having a front surface adjacent to said path so as to periodically sharpen said cutting edge during movement of said carriage; and reconditioning means including a dresser pivotable about a predetermined axis to thereby recondition said front surface, said dresser being adjacent to one axial end of said holder and said carriage being arranged to dwell in a predetermined position at said one end of said holder between successive movements thereof while said dresser pivots about the respective axis to thereby treat said front surface, said predetermined axis being at least substantially parallel with the axis of said holder and the shortest distance between said predetermined axis and the axis of said holder exceeding the radius of said path.
 5. A combination as defined in claim 2, wherein said predetermined axis is fixed.
 6. A combination as defined in claim 2, wherein said predetermined axis is and crosses in space with normal to the axis of said grinding wheel.
 7. In an apparatus of the character indicated, particularly in a tobacco shredding machine, a combination comprising a rotary holder; at least one knife mounted in said holder and having an exposed cutting edge arranged to travel along a circular path in response to rotation of said holder; a carriage movable in parallelism with the axis of said holder; a grinding wheel rotatably mounted in said carriage and having a front surface adjacent to said path so as to periodically sharpen said cutting edge during movement of said carriage; reconditioning means including a dresser pivotable about a predetermined axis to thereby recondition said front surface, said dresser being adjacent to one axial end of said holder and said carriage being arranged to dwell in a predetermined position at said one end of said holder between successive movements thereof while said dresser pivots about the respective axis to thereby treat said front surface; anD synchronizing means for effecting pivotal movements of said dresser in response to movement of said carriage to said predetermined position, said front surface of said grinding wheel being disengaged from said cutting edge in said predetermined position of said carriage.
 8. A combination as defined in claim 1, further comprising a second carriage movable in parallelism with the axis of said holder and a second grinding wheel rotatably mounted on said second carriage and having a conical active surface adjacent to said path so as to periodically sharpen said cutting edge during movement of said second carriage.
 9. In an apparatus of the character indicated, particularly in a tobacco shredding machine, a combination comprising a rotary holder; at least one knife mounted in said holder and having an exposed cutting edge arranged to travel along a circular path in response to rotation of said holder; a first carriage movable in parallelism with the axis of said holder; a first grinding wheel rotatably mounted in said carriage and having a front surface adjacent to said path so as to periodically sharpen said cutting edge during movement of said carriage; a second carriage movable in parallelism with the axis of said holder; a second grinding wheel rotatably mounted in said second carriage and having a front surface adjacent to said path so as to periodically sharpen said cutting edge during movement of said second carriage; and discrete reconditioning means for each of said grinding wheels, each of said reconditioning means including a dresser pivotable about a predetermined axis to thereby recondition the front surface of the respective grinding wheel, said dressers being adjacent to the opposite axial ends of said holder and said carriages being arranged to dwell in predetermined positions at the respective axial ends of said holder between successive movements thereof while said dressers pivot about the respective axes to thereby treat the corresponding front surfaces.
 10. A combination as defined in claim 9, further comprising coupling means connecting said carriages for simultaneous movement with reference to said holder.
 11. A combination as defined in claim 1, wherein the radius of curvature of said active surface at least equals the radius of said circular path. 